Question

I have found the following method in an open source project:

    static IEnumerable<T> Cat<T>(T t) {
        yield return t;
    }

As far as I understand, it does nothing else than returning an IEnumerable<T> containing t. If I am right and it’s virtual equivalent to e.g. return new List<T>{t};, what’s the purpose/advantage of using yield return here? And if I am wrong, what do I miss?

Full source can be found here.

Answer 1

This just makes an enumerable type that contains a single element (t). In practical terms, you are correct – it’s effectively going to be very similar to just returning a new List<T> with one element, or a new array with one element, etc.

The main difference is that the returned type won’t actually be a List<T> or an array – it’ll be a compiler generated type. You wouldn’t be able to cast the result to a list and add elements, for example. In practical purposes, it’ll behave identically to returning a new List<T> as you described.

There is likely no real advantage, other than the developer probably preferred this syntax. I suspect was probably used to allow single elements to work with other portions of the API which expect an IEnumerable<T> to be passed in, though the method name leaves a bit to be desired, IMO.